The invention relates more particularly to channel estimation (i.e., sounding), or the estimation of the impulse response of a transmission channel transporting information from a transmitter to a receiver via a propagation medium. This propagation medium may be air in the case of cellular mobile telephones, or any other propagation medium (e.g., a cable) in other applications.
One fundamental factor limiting the performance of a digital communication system is the phenomenon well known to the person skilled in the art as inter-symbol interference. Such inter-symbol interference within the receiver causes each symbol transmitted (e.g., a bit) to take up an amount of time greater than the initial duration of the symbol (i.e., the bit time). In other words, the signal received at a given instant depends not only on a single symbol (e.g., bit), but also on the other symbols (bits) sent which extend over durations longer than those of one symbol (bit time).
In practice, the signal received at a given instant depends upon the symbol in question as well as on the adjacent symbols. Causes of the inter-symbol interference may be numerous. One such cause is due especially to the multiple propagations of the signal between the transmitter and the receiver when the signal is reflected or diffracted by various obstacles. This leads, upon reception, to several copies of the signal mutually shifted in time.
During communications with interference between symbols, it may be difficult to estimate the impulse response of the transmission channel. The quality of this estimate depends upon the capability of eliminating the interference between symbols, and thus of making the correct decisions as to the symbols sent. In general, the estimate of the impulse response of the channel, or more simply the channel estimate, is produced by using least-squares techniques and by using a predetermined sequence of symbols known to the transmitter and to the receiver. This is often referred to as the “training sequence.” This may particularly be the case with respect to GSM-based telephones.
The training sequence is present within each burst of symbols sent. When the characteristics of the channel have been sufficiently well estimated, the estimated coefficients of the impulse response of the channel are used in equalization processing, as will be appreciated by those of skill in the art. This is to decipher the received signal, i.e., to recover the logic values of the symbols (data) sent in the burst.
The equalization processing is conventionally followed by processing operations called channel decoding, which are intended to correct any errors to the extent possible. The channel decoding is itself conventionally followed by source decoding, which is intended to reconstitute the information (e.g., speech) initially coded within the transmitter. As noted above, the reception quality, which is generally expressed by the bit error rate (BER), depends greatly on the quality of estimation of the channel.
One of the important parameters for the quality of the estimation lies in the spreading of the channel. This spreading of the channel represents the duration of the channel response to be estimated, and it fixes the number of coefficients of the impulse response of the channel. In practice, depending upon the environment in which the telephone is located, the paths may be more or less spread. Thus, the most spread path is encountered in hilly environments and 7 to 8 coefficients may be then necessary to estimate the impulse response of the channel correctly.
In contrast, there exist paths with less pronounced spreading such as the paths called static paths, which are direct paths with no reflection. For such static paths, only 4 to 5 coefficients may be necessary for a good estimate of the impulse response of the transmission channel. Furthermore, urban paths, which represent the most frequent cases for cellular mobile telephony systems, typically require 5 to 6 coefficients to estimate the impulse response of the transmission channel correctly.
The estimation of the impulse response of the channel is carried out at regular intervals. To produce this estimate the length of the channel is currently defined in advance, i.e., a number of coefficients are fixed a priori for the impulse response of the channel. By way of example, in the case of GSM, the number of coefficients is fixed at a maximum value to satisfy the most stringent recommendations, such as those set forth in the standard ETSI EN 300 910 V8.5 (July 2000) entitled Digital cellular telecommunications systems (Phase 2+), Radio transmission and reception (GSM 05.05 Version 8.5.0 Release 1999).
In other words, the number of coefficients of the impulse response of the transmission channel is taken to be that corresponding to the path exhibiting the most pronounced spreading, i.e., a hilly terrain. However, fixing the value of the number of coefficients of the impulse response of the channel at its maximum value makes it possible to be successful in equalization for the channels which are the most spread. Yet, this substantially degrades performance for the channels with slight spreading, such as those which are encountered in urban environments.